This invention relates to a memory device and, in particular, to a magnetic random access memory (hereinafter abbreviated to MRAM) using a nonvolatile magnetic memory cell.
At present, development of an MRAM is making rapid progress as a large-capacity memory device capable of carrying out high-speed reading and writing operations. As the MRAM, use is predominantly made of a structure comprising a number of memory cells and utilizing so-called tunnel junction. The tunnel junction makes use of the fact that an electric resistance between two magnetic layers with a nonmagnetic film interposed therebetween is different depending upon whether spins known in the art are parallel or anti-parallel to each other between the magnetic layers. The MRAM of the type includes a transistor for selecting a specific memory cell to be accessed.
Referring to FIG. 1, description will be made of an existing MRAM. The MRAM illustrated in FIG. 1 comprises a substrate 1 and an integrated device portion 2 mounted on the substrate 1. The integrated device portion 2 is connected through lead wires 3 to lead terminals 4. The substrate 1, the integrated device portion 2, the lead wires 3, and the lead terminals 4 are molded by a resin mold 5.
The integrated device portion 2 includes a number of transistor portions and a number of memory device portions. Each of the memory device portions serves as a memory cell. Each of the transistor portions serves to select a particular memory cell.
Referring to FIG. 2, description will be made of the structure of the integrated device portion 2. The integrated device portion 2 comprises a first conductor 11 on the substrate 1, a first insulator layer 12 on the first conductor 11, a first ferromagnetic member or layer 13 on the first insulator layer 12, a second insulator layer 14 covering the first ferromagnetic member 13, a second ferromagnetic member 15 on the second insulator layer 14, a third insulator layer 16 covering the second ferromagnetic member 15, and a fourth insulator layer 17 and a second conductor 18 both of which are formed on the third insulator layer 16. A combination of the first ferromagnetic member 13, the second insulator layer 14, and the second ferromagnetic member 15 forms a magnetic tunnel function device as one of the memory device portions.
Each of the first and the second conductors 11 and 18 is arranged so that the second ferromagnetic member 15 is applied with a magnetic field when an electric current is supplied thereto. In case where both of the first and the second conductors 11 and 18 are supplied with electric currents, magnetic fields are produced by the electric currents and combined into a composite magnetic field. Under the composite magnetic field, magnetization of the second ferromagnetic member 15 is rotated and reversed. On the other hand, the first ferromagnetic member 13 is fixed in magnetization, for example, by the use a ferromagnetic material having high saturation magnetization.
The first ferromagnetic member 13 is made of a CoPt alloy while the second ferromagnetic member 15 is made of a NiFe alloy. The second insulator layer 14 is made of Al2O3 or the like.
In the meanwhile, highly-integrated semiconductor devices capable of carrying out high-speed operations include not only the MRAM but also a dynamic random access memory (DRAM), a read-only memory (ROM), a microprocessor unit (MPU), and an image processor arithmetic logic unit (IPALU), and so on. Recently, these devices are remarkably increased in calculation speed and signal processing speed. Such increase in calculation speed and signal processing speed results in drastic or quick change in electric current flowing in these devices. The quick change in electric current is a major factor causing high-frequency inductive noise.
On the other hand, reduction in weight, thickness, and size of electronic components and electronic apparatuses is making rapid progress also. Therefore, the degree of integration of the semiconductor device as an electronic component and the density of mounting the same on a printed wiring board are increased. As a consequence, if the electronic component is highly integrated or the electronic components are mounted at a high density, signal lines are very close to each other. In combination with the above-mentioned increase in signal processing speed, high-frequency radiation noise is readily induced.
In the above-mentioned electronic circuit, an attempt to suppress the noise has been made by optimizing the design in arrangement of components on the printed wiring board and wiring therebetween or by inserting a lumped-constant component such as a decoupling capacitor into a power supply line.
However, in the semiconductor device or the printed wiring board increased in operation speed, the noise generated therefrom contains harmonics components so that the behavior of a signal path is similar to that of a distributed-constant circuit. In this event, the existing noise countermeasure assuming a lumped-constant circuit is no longer effective. In addition, limitation is imposed upon reduction of the noise by optimizing the arrangement of the electronic parts and the wiring.
In the operation of the above-mentioned MRAM, harmonic distortion produced upon high-speed change in electric current is a major factor causing the high-frequency radiation noise, like other types of semiconductor random access memories (RAM). On the other hand, the noise superposed on a writing current or a magnetic layer causes the fluctuation in magnitude of magnetization of the magnetic layer. As a result, an additional operation will be required upon writing. If the noise is mixed in a signal upon reading, an additional process such as repetition of a reading operation is required. In other words, if the noise countermeasure is not effective, substantial writing and reading speeds are decreased upon writing and reading data. Therefore, in the operation of the MRAM, it is important not only to prevent the noise from being propagated to other components or portions but also to prevent reduction in substantial writing and reading speeds due to the noise.
It is therefore an object of this invention to provide a large-capacity non-volatile memory which is suppressed in generation of noise and is excellent in noise resistance so as to carry out writing and reading operations at a substantially high speed.
Other objects of the present invention will become clear as the description proceeds.
The present inventors have already invented a composite magnetic material having a high magnetic loss at a high frequency and found out a method of effectively suppressing extraneous emission or unnecessary radiation produced from a semiconductor device or an electronic circuit by arranging the composite magnetic material in the vicinity of an extraneous emission source. From the recent research, it has been found out that the effect of attenuating the extraneous emission utilizing the magnetic loss is based on a mechanism in which an equivalent resistance component is added to the electronic circuit as the extraneous emission source. Herein, the magnitude of the equivalent resistance component depends upon the magnitude of a magnetic loss term xcexcxe2x80x3 of a magnetic material. Specifically, the magnitude of the resistance component equivalently inserted into the electronic circuit is substantially proportional to xcexcxe2x80x3 and the thickness of the magnetic material as far as the area of the magnetic material is fixed. It is noted here that the magnetic loss term xcexcxe2x80x3 is an imaginary part of the relative permeability of the magnetic material. Therefore, in order to achieve a desired level of attenuation of the extraneous emission by the use of a smaller or a thinner magnetic material, the value of xcexcxe2x80x3 must be greater. For example, in order to prevent the extraneous emission utilizing a magnetic loss material in a very small region such as an interior of a mold of the semiconductor device, the magnetic loss term xcexcxe2x80x3 must have a very large value.
During the research of a soft magnetic material produced by sputtering or vapor deposition, the present inventors have focused upon excellent permeability characteristics of a granular magnetic material uniformly distributed with fine magnetic metal particles or granules in a nonmagnetic material such as ceramics. As a result of investigating a fine structure of the magnetic metal particles surrounded by a nonmagnetic material, the present inventors have found out that, in case where the ratio of the magnetic metal particles in the granular magnetic material fall within a specific range, excellent magnetic loss characteristics are achieved in a high-frequency band.
It is noted here that a granular magnetic thin film is a magnetic thin film in which magnetic particles have a size as small as several nanometers to several tens nanometers and each particle has a fine structure bounded by a grain boundary comprising a ceramics component and which exhibits a very large magnetic loss in a high-frequency band between several tens MHz to several GHz. The granular magnetic thin film may be called a fine crystal thin film.
As regards the granular magnetic material having a composition represented by M-X-Y (M being a magnetic metal element, Y being one element selected from oxygen, nitrogen, and fluorine, X being an element other than M and Y), a number of researches have been made so far. It is revealed that the granular magnetic material has a low magnetic loss and a great saturation magnetization. In order to achieve a greater saturation magnetization in the M-X-Y granular magnetic material, it is necessary to increase the ratio of the component M. Therefore, in general applications such as a high-frequency inductor device or a magnetic core of a transformer, the ratio of the component M in the M-X-Y granular magnetic material is limited to a range such that the saturation magnetization is equal to about 80% or more of the saturation magnetization of a bulk metal magnetic material comprising the component M alone.
The present inventors investigated over a wide range of the ratio of the component M in the granular magnetic material having a composition represented by M-X-Y (M being a magnetic metal element, Y being one element selected from oxygen, nitrogen, and fluorine, X being an element other than M and Y). As a result, it has been found out that, in any composition system, a large magnetic loss is exhibited in a high-frequency band if the concentration of the magnetic metal M falls within a specific range.
A highest region of the ratio of the component M is such that the saturation magnetization is 80% or more of the saturation magnetization of a bulk metal magnetic material comprising the component M alone. This region corresponds to the M-X-Y granular magnetic material which has high saturation magnetization and low loss and which has been actively researched and developed. Those materials within the above-mentioned region are large in both of real part permeability xcexcxe2x80x2 and saturation magnetization and are therefore used in high-frequency micromagnetic devices such as the above-mentioned high-frequency inductor. However, the ratio of each of the components X and Y determining electric resistance is small so that the electric resistance is small. Therefore, if the thickness of the film is increased, the permeability at the high-frequency is degraded following occurrence of eddy current loss at the high-frequency band. Thus, these materials are inappropriate as a magnetic film for a noise countermeasure.
A next region of the ratio of the component M is such that the saturation magnetization is not greater than 80% and not smaller than 60% of the saturation magnetization of the bulk metal magnetic material comprising the component M alone. In this region, the electric resistance is equal to about 100 xcexcxcexa9xc2x7cm which is relatively large. Therefore, even if the thickness of the material is on the order or several xcexcm, the eddy current loss is small and most of the magnetic loss results from natural resonance. As a consequence, the magnetic loss term xcexcxe2x80x3 has a narrow width of frequency distribution. Thus, this region is appropriate for the noise countermeasure (high-frequency current suppression) in a narrow frequency band.
A third region of the ratio of the component M is such that the saturation magnetization is not greater than 60% and not smaller than 35% of the saturation magnetization of the bulk metal magnetic material comprising the component M alone. In this region, the electric resistance is equal to about 500 xcexcxcexa9xc2x7cm which is greater than that mentioned above. Therefore, the eddy current loss is extremely small and magnetic interaction between particles of the component M is small. Therefore, thermal disturbance of the spin is increased and the frequency causing the natural resonance is fluctuated. As a result, the magnetic loss term xcexcxe2x80x3 exhibits a large value over a wide frequency band. Thus, this region of the ratio is appropriate for suppression of the high-frequency current over a wide band.
On the other hand, a region of a smaller ratio of the component M provides ultramagnetic characteristic because no substantial magnetic interaction between the particles of the component M is caused to occur.
In case where the magnetic loss material is arranged in the vicinity of a noise radiating portion to suppress the high-frequency current, the standard of material design is given by a product xcexcxe2x80x3xc2x7xcex4 of the magnetic loss term xcexcxe2x80x3 and the thickness xcex4 of the magnetic loss material. In order to achieve effective suppression of the high-frequency current of several hundreds MHz, the following relationship must be satisfied:
xcexcxe2x80x3xc2x7xcex4xe2x89xa7about 1000 (xcexcm)
Specifically, if the magnetic loss material has a magnetic loss term xcexcxe2x80x3=1000, the thickness must be equal to 1 xcexcm or more. Those material having a low electric resistance and easy to cause eddy current is not preferable. The composition such that the electric resistance is 100 xcexcxcexa9xc2x7cm or more is preferable. In the composition system used in this invention, it is preferable that the ratio of the component M falls within the region such that the saturation magnetization is not greater than 80% and not smaller than 35% of the saturation magnetization of the bulk metal magnetic material comprising the component M alone. The region such that the saturation magnetization is equal to 35% or more of the saturation is a region where no ultramagnetism is exhibited.
The present inventors have made this invention by applying the above-mentioned magnetic material to the MRAM.
According to the present invention, there is provided a magnetic random access memory which comprises a memory device portion using magnetic material and a high-frequency current suppressor arranged in the vicinity of the magnetic material for suppressing a high-frequency current which flows in the memory device portion.